I am trying to test sample code for web extension for browsers.
But, it doesn't work. I checked the console for Google Chrome and also for Firefox. It doesn't print anything. The following is my code:

manifest.json:

{

    "description": "Demonstrating webRequests",

    "manifest_version": 2,

    "name": "webRequest-demo",

    "version": "1.0",



    "permissions": [

        "webRequest"

    ],

    "background": {

        "scripts": ["background.js"]

    }

}

background.js:

function logURL(requestDetails) {

    console.log("Loading: " + requestDetails.url);

}



chrome.webRequest.onBeforeRequest.addListener(

    logURL,

    {urls: ["<all_urls>"]}

);

console.log("Hell o extension background script executed");

Am I missing something?

Firefox

In Firefox, your code works (outputs to console), as it is written in the question.

If you are not seeing it in the console, then you are, probably, looking at the wrong console.

Mozilla describes what extension output can be seen in which console in their Debugging page.

Browser Console

You should be using the Browser Console. You can access it from Tools➜Web Developer➜Browser Console (keyboard shortcut Ctrl-Shift-J, or Cmd-Shift-J on Mac).

Browser Toolbox

If you have it enabled, you could also use the Browser Toolbox console. You can access it from Tools➜Web Developer➜Browser Toolbox (keyboard shortcut Ctrl-Alt-Shift-I; On a Mac: Cmd-Alt-Shift-I).

Add-on Debugger

To debug your add-on you can use the Add-on Debugger. You can access it though about:debugging➞Debug.

Web Console

You, probably, are looking at the Web Console (keyboard shortcut F12) which is associated with only a single tab. This is what you want when debugging a webpage, but not an add-on. For a content script which is injected in that tab, the console.log() output will show up in this console. However, you will not see output from any other portion of your add-on (e.g. not content scripts in other tabs, not background scripts, etc.).

Google Chrome

Showing the correct console for your extension is a bit more complex in Chrome. Console output will show up in only one of multiple possible places, depending on from what context the console.log() was executed. Each of the following DevTools are independent of each other and are displayed in separate windows, or tabs. Displaying in the associated tab (bottom or side) is the default for the DevTools associated with web pages and content scripts, because those are specific to the tab. For the web page/content script DevTools, you have the option of having it displayed in its own separate window, or docked inside the tab (side or bottom).

For your Background page

As Srujan Reddy explained, you have to go through multiple selections on a drop-down menu, to get to the chrome://extensions page (or you can type that in by hand as the URL, or use a bookmark) then select both a checkbox ("Developer mode") and then click on the "background page" link. Then, you have to select the "Console" tab on the window that pops up.

It is much easier to show what you have to do:

For your Content Scripts

Output will be shown in the regular web console (in the web Developer Tools). You can open it by pressing F12 (or other shortcuts) in the webpage in which your content script was injected. Each web console will only show the output from the scripts injected in that tab.

Doing the above with show the console.* output from your extension, but will result in the console JavaScript command line, debugger, etc. being in the context of the page, not the content script.

If you want to use the console JavaScript command line in the context of the content scripts which are injected into a web page, you need to select your extension's content script context from the drop-down menu in the upper left of the Console window. This drop-down menu will normally start with the value "top". The drop down will have selections for each of the content script contexts (one per extension that has script(s) injected).

For your Popup

Right-click on your browserAction button and select "Inspect Popup". Alternately, right-click within the popup and select "Inspect". Either will open the DevTools for the popup page. The popup will be kept open under more conditions than it normally would, but will still be closed if you switch tabs, etc.

For your Options page

Right-click within the main content of the Options popup (not the title bar) and select "Inspect". This will open the DevTools for the options page.

For your panel or a page from within your extension loaded in a tab

When the panel or tab is focused, you can open the DevTools by pressing F12 (or other shortcuts), or by opening the context menu (right-click) and selecting "Inspect".

Which console are you viewing in for the logs?

If you are viewing console on a tab, then that is the wrong place.

Open settings / Extensions or in a new tab type

chrome://extensions

Under your extension click on "background page" link which is where you can watch for logs

**Make sure Developer mode is checked

Yes for Chrome you have to click the background page link mentioned, but it won't be there unless you set it to be persistent:

"background": {

    "persistent": true,

    "scripts": ["background.js"]

}